In the given figure,what is the structure labeled as $A$?

$S$ - Statement: $RNA$ and protein molecules move between the nucleoplasm and the cytoplasm.
$R$ - Reason: Nuclear pores are present at several locations in the nuclear envelope of the nucleus.

Which of the following statements is incorrect in relation to the nucleolus?

Where is ribosomal $-RNA$ actively synthesized?

The nucleolus in eukaryotic cells is:

Which organelle is known as the control center of the cell?